1. Field of Invention
The present invention relates to a method of driving a liquid crystal display device, a liquid crystal display device, and a portable electronic apparatus, and more specifically, it relates to a common inversion driving method of a liquid crystal display device using an active matrix substrate.
2. Description of Related Art
Recently, liquid crystal display devices using active elements such as a thin film transistor have been widely used in the fields of a notebook PC or a monitor. In the liquid crystal display device using typical nematic liquid crystal materials, it is necessary to adopt an alternating current driving method in which a polarity of a voltage to be applied to the liquid crystal is inverted for every predetermined time in order to secure reliability. Generally, a difference in voltage to be applied to the liquid crystal for a white display and a black display is in a range of 3 through 5 V. Therefore, in order to implement the alternating current driving method, when the fixed potential are applied to a electrode (common electrode) of a substrate opposing an active matrix substrate with liquid crystal interposed therebetween, a signal having a voltage amplitude of 6 through 10 V should be applied to a pixel electrode on the active matrix substrate. However, since a process of high pressure resistance is required to output a signal having a voltage amplitude of 5 V or more from a typical IC (integrated circuit), a manufacturing cost increases. To avoid this problem, there has been proposed a common inversion driving method in which input signals are decreased by alternating-current driving the potential of the common electrode (see, Patent Document 1).
Now, a 1H common inversion driving method executing a common inversion and a polarity inversion of the voltage applied to the liquid crystal for every scanning line selection cycle (1H cycle) will be described with reference to FIG. 12. Herein, it is supposed that the liquid crystal display device operates, for example, in a normally white mode and has an N-channel thin film transistor as a pixel switching element.
A reference symbol Vcom(1) denotes a potential of the common electrode, and when an auxiliary capacitor Cs is formed, an auxiliary capacitor common electrode also has the same value. The Vcom(1) is periodically inverted between potentials VcomH and VcomL in the case of the common inversion driving method. In addition, a reference symbol VG1 to n(2-1 through 2-n) denotes a potential applied to the nth scanning line from a scanning line driving circuit. For every inversion of the Vcom(1), a selection potential VGON is sequentially applied to one scanning line for turning on the pixel switching element. At other times, one of the potentials VGOFFH and VGOFFL is selected according to the potential Vcom(1) and then applied as a non-selection signal for turning off the connected pixel switching element. Herein, the non-selection signal has two different levels VGOFFH and VGOFFL according to the potential Vcom(1) in order to secure reliability of the pixel switching element. This is disclosed in, for example, Patent Document 2 in detail. A reference symbol VS1 to m(3-1 through 3-m) denotes a video signal potential applied to a data line from a data line driving circuit, and has an amplitude between VVIDEOH and VVIDEOL. If the liquid crystal material or the gap is selected such that a white (transparent) display is implemented when the liquid crystal element is interposed between electrodes having a potential difference of ±VWHITE, and a black (non-transparent) display is implemented when the liquid crystal element is interposed between electrodes having a potential difference of ±VBLACK, it is possible to obtain VcomH≧VVIDEOH>VVIDEOL≧VcomL, and VcomH−VVIDEOH=VVIDEOL−VcomL=VVWHITE, VcomH−VVIDEOL=VVIDEOH−VcomL=VBLACK.
The potential of VS1 to m(3-1 through 3-m) is applied to the pixel electrode through the pixel switching element connected to the scanning line having a selection potential VGON. Herein, if VPIX4-1-1 through VPIX4-n-m denotes potentials of the pixel electrodes connected between an mth data line and an nth scanning line, VPIX4-1-1 and VPIX4-1-2 are charged with the potentials VS1 and VS2 of the data lines 1 and 2, respectively, and become potentials VVIDEOH and VVIDEOL when the scanning line 1 is the selection potential VGON. In this case, the common potential is VcomH, and to the liquid crystal on the pixel electrode corresponding to VPIX4-1-1, a potential of VVIDEOH−VcomH=−VWHITE is applied.
The potential of VS1 to m(3-1 through 3-m) is applied to the pixel electrode through the pixel switching element connected to the scanning line having a selection potential VGON. Herein, if VPIX4-1-1 through VPIX4-n-m denotes potentials of the pixel electrodes connected between an mth data line and an nth scanning line, the potentials VPIX4-1-1 and VPIX4-1-2 are charged with the potentials VS1 and VS2 of the data lines 1 and 2, respectively, and become potentials VVIDEOH and VVIDEOL when the scanning line 1 is the selection potential VGON. In this case, the common potential is VcomH, and to the liquid crystal on the pixel electrode corresponding to VPIX4-1-2, a potential of VVIDEOL−VcomH=−VBLACK is applied. In other words, the pixel corresponding to VPIX4-1-1 is subjected to a transparent (white) display, and the pixel corresponding to VPIX4-1-2 is subjected to a non-transparent (black) display.
Subsequently, the common potential is inverted to VcomL when the scanning line 2 is selected, the pixel electrodes corresponding to VPIX4-1-1 and VPIX4-1-2, respectively, are in the floating state because the switching electrode is a high resistance state. Therefore, supposing that capacitive elements except for the common electrode and the capacitor line are negligible, the potentials VPIX4-1-1 and VPIX4-1-2 are simultaneously dropped by the amount of change of the potential (VcomL−VcomH) of the common electrode due to the capacitive coupling. As a result, the pixel corresponding to VPIX4-1-1 maintains the transparent (white) display, and the pixel corresponding to VPIX4-1-2 maintains the non-transparent display (black). As described above, even though the common potential is repeatedly inverted, the potential difference from the pixel electrode connected to the scanning line of the non-selection potential is not altered. Therefore, the same grayscale display can be maintained until the next scanning line becomes the selection potential.
On the other hand, VPIX4-2-1 and VPIX4-2-2 are charged with the potentials VS1 and VS2 of the data lines 1 and 2 when the scanning line 2 is the selection potential (VGON), and become potentials VVIDEOL and VVIDEOH, respectively. In this case, a potential of VVIDEOL−VcomL=VWHITE is applied to the liquid crystal on the pixel electrode corresponding to the VPIX4-2-1 and a potential of VVIDEOH−VcomL=VBLACK is applied to the liquid crystal on the pixel electrode corresponding to the VPIX4-2-2, so that a transparent (white) display and a non-transparent (black) display are implemented, respectively. However, they have voltage polarities opposite to those of the pixels corresponding to VPIX4-1-1 and VPIX4-1-2. Similarly to the above description, though the common potential is inverted after the scanning line 2 becomes a non-selection potential, the potential difference between the common potential and the pixel potential is not altered, so that the display is retained. When the scanning line becomes the selection potential again in the next frame after the rewriting time according to a refresh rate, the common potential is VcomL if the scanning line 1 becomes the selection potential VGON, and the common potential is VcomH if the scanning line 2 becomes the selection potential VGON. Moreover, a polarity of the potential across the liquid crystal element is inverted with respect to the previous frame. Therefore, an alternate driving of the liquid crystal can be implemented. Until now, the conventional 1H common inversion driving method has been described.
According to this method, the amplitude of the input video signal from an external IC is 3 through 5 V. Therefore, it is possible to use a commercial IC made by typical CMOS processes, a manufacturing cost can be reduced. This is the same because an IC for outputting video signals is necessary in the case of an analog driving method in which video analog signals are inputted, even when driving circuits of the active matrix substrate are provided externally, as well as when the driving circuits are embedded in the active matrix substrate, and a power source IC for supplying DC power source to a DAC or a decoder is necessary in the case of a digital driving method in which the DAC or the decoder is embedded. In addition, the common inversion driving method is an effective method even in the case of a power source and driving circuit embedded LCD in which the power generating circuit is embedded in the active matrix substrate, since the circuit size and the current consumption increase and the reliability of the thin film transistor is badly influenced as the voltage range of the generated power source becomes wider, [Patent Document 1] Japanese Unexamined Patent Application Publication No. 62-49399 [Patent Document 2] Japanese Unexamined Patent Application Publication No. 2001-306041